Shock absorbing material

ABSTRACT

A shock absorbing material comprising an Al—Mg—Si series aluminum alloy having high strength and showing excellent energy absorbing property when compressed in the axial direction of extrusion is obtained. The shock absorbing material of the invention has a hollow cross section, mainly comprises a fibrous structure and can be manufactured by press quenching just after extrusion followed by aging. In the press quenching, press quenching under air-cooling advantageous in view of the dimensional accuracy or the cost can be adopted. Further, the shock absorbing material of the invention has excellent cracking resistance to a compressive load in the lateral direction as well as in the axial direction. The shock absorbing material of the invention is suitable as side members or bumper stays in the frame structures of automobiles.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention concerns a shock absorbing materialcomprising an Al—Mg—Si series aluminum alloy extrusion material andhaving a function of absorbing impact loads when undergoing compressiveimpact loads. The shock absorbing material according to the presentinvention is suitable, for example, to side members, bumper stays orside frames in frame structures of automobiles.

[0003] 2. Related Art

[0004] In the frame structures for automobiles, it has been consideredthe use of aluminum alloy hollow extrusion materials in view ofreduction of weight as shock absorbing materials such as side members orbumper stays. The shock absorbing materials undergoing axial compressiveimpact loads are required not to suffer from Eulerian buckling for theentire shape material (buckling in which the entire shape material isbent in an angled form) when undergoing loads in the axial direction ofextrusion, to cause shrinking deformation in a bellows form with nocracks to provide stable high energy absorption, and have requiredstrength (yield stress) as frame structural materials for automobiles.

[0005] As aluminum alloy extrusion materials usable as the shockabsorbing material, various Al—Mg—Si series aluminum alloy extrusionmaterials which are relatively excellent in corrosion resistance amonghigh strength aluminum alloys and excellent over other series aluminumalloys also in view of recycling performance have been studied so far(for example, Japanese Published Unexamined Patent Application Hei6-25783, Japanese Published Unexamined Patent Application Hei 7-54090,Japanese Published Unexamined Patent Application Hei 7-118782 andJapanese Published Unexamined Patent Application Hei 9-256096).

[0006] As described in the publications cited above, when Al—Mg—Siseries aluminum alloy extrusion materials are used for the shockabsorbing materials, press quenching in on-line or solution/quenchingtreatment in off-line is generally applied followed by aging. The agingtreatment is applied for improving the strength of the extrusionmaterials and stabilizing the microstructure to prevent progress ofspontaneous aging which deteriorates the cracking resistance during use.

[0007] Press quenching under water-cooling has an advantage capable ofproviding substantially equal characteristics with those obtained by thesolution/quenching treatment of applying reheating after extrusion.However, this results in a difference in the cooling rate along thecross section due to the difference of the cross sectional shape or thewall thickness of the extrusion materials, and makes the temperaturedistribution not uniform during cooling to cause distortion.Accordingly, it results in problems that the accuracy is poor, thicknessfor the cross sectional shape is difficult to be reduced and the degreeof freedom for the cross sectional shape is restricted if it is intendedto prevent occurrence of such distortion. There is also an additionalproblem that it requires higher cost compared with that in air-cooling.

[0008] On the other hand, press quenching under air-cooling has a meritof reduced cost compared with press quenching under water-cooling.However, since the cooling rate is limited, it results in a problem thatno high strength (particularly, yield stress) can be obtained dependingon the alloy compositions and, even when a high strength can beobtained, the energy absorption or the cracking resistance is poor.

[0009] In view of the above, it is an object of the present invention toprovide a shock absorbing material comprising an Al—Mg—Si seriesaluminum alloy suitable to press quenching under air cooling and capableof obtaining high strength.

[0010] Further, Al—Mg—Si series aluminum alloy extrusion materialsinvolve a worry that spontaneous aging progresses to deteriorate thecracking resistance when exposed at a high temperature during usethereof, for example, as side members. Aging treatment is applied as anessential condition to prevent this in a case of using heat treated typeAl—Mg—Si series aluminum alloy extrusion materials as a shock absorbingmaterials. However, when strength is improved by T5, T6 treatment, itinvolves a problem of causing cracks when subjected to axial compressivedeformation. If cracks should occur, shrinking deformation in thebellows-form is inhibited failing to obtain stable energy absorption.

[0011] Further, it has been demanded recently that Al—Mg—Si seriesaluminum alloy extrusion materials can be utilized also as other framestructural materials for automobiles such as side frames that undergolateral impact loads with a view point of recycling use.

[0012] In view of the above, it is a second object of the presentinvention to provide an Al—Mg—Si series aluminum alloy extrusionmaterial having high strength (yield stress) with excellent crackingresistance in the axial direction and energy absorbing property and, atthe same time, excellent cracking resistance also in the lateraldirection.

[0013] The present inventors have found an alloy composition and astructure having high strength (yield stress) and showing excellentenergy characteristics when compressed in the axial direction ofextrusion (showing high energy absorption with no occurrence of cracks)for the shock absorbing material of an Al—Mg—Si series aluminum alloybased on the premise of adopting press quenching under air cooling,which is advantageous in view of the dimensional accuracy and the cost.

SUMMARY OF THE INVENTION

[0014] The shock absorbing material according to the first inventioncomprises an Al—Mg—Si series aluminum alloy which contains from 0.30 to0.70 wt % of Mg from 0.10 to 0.50 wt % of Si in excess of the balancedcomposition for Mg₂Si and from 0.10 to 0.40 wt % of one or more of Mn,Cr and Zr in total, and has a fibrous structure, and has an yield stressof 210 MPa or more, which is subjected to press quenching under aircooling and then aging after extrusion into a hollow cross sectionalshape.

[0015] The Al—Mg—Si series alloy optionally contains from 0.005 to 0.2%of Ti, from 0.10 to 0.40% of Cu and the like and contains Fe and otherelements as inevitable impurities.

[0016] In the Al—Mg—Si series aluminum alloy extrusion materials, theaddition amount of Mg—Si and transition elements (such as Cu, Mn, Cr andZr) is generally increased if it is intended to increase the strengthafter the aging treatment, in which precipitates inevitably increase atthe grain boundaries to concentration strain to the grain boundaries.Further, the quenching sensitivity is generally increased as the amountof the additive elements increases and the amount of the precipitates atthe boundaries also increases at a low cooling rate (for example, in aircooled press quenching which is advantageous in view of the dimensionalaccuracy or the cost), tending to cause cracks when crushed under largeimpact loads.

[0017] On the contrary, the present inventors have found that Al—Mg—Siseries aluminum alloy extrusion materials tend to cause cracks if themacrostructure thereof mainly comprises a recrystallization structurebut occurrence of such cracks can be suppressed even at a high strengthif it mainly comprises a fibrous structure (in a state where the fibrousstructure after extrusion is left as it is with no recrystallizationalso during a heat treatment step subsequent to the extrusion step). Onthe other hand, it has also been found that the fibrous structure isloosened when the solution quenching treatment is applied in off-lineand that formation of a surface recrystallization layer to a certainthickness is advantageous in order to obtain required characteristics byon-line press quenching.

[0018] The second invention has been accomplished based on the findingdescribed above.

[0019] The Al—Mg—Si series aluminum alloy shock absorbing materialaccording to the present invention comprises an Al—Mg—Si series aluminumalloy of a hollow cross section containing from 0.20 to 1.1 wt % of Mgand from 0.20 to 1.2 wt % of Si, which is aged after press quenchinghaving an yield stress of 200 MPa or more and a surfacerecrystallization layer with a thickness of 1 to 50% for the entire partand the crystal grain size in the direction of the thickness of thesurface recrystallization layer is 200 μm or less.

[0020] The fibrous structure layer is present on the inside of thesurface recrystallization layer of the extrusion material.

[0021] In the course of various experiment and studies for developingthe aluminum alloy extrusion material of excellent cracking resistance,the present inventors have found that the reduction rate of thicknessalong the ruptured cross section of a JIS No. 5 tensile test piecesampled from an extrusion material of a hollow cross section is closelyrelated with the cracking resistance of the extrusion material and haveaccomplished the third invention.

[0022] That is, the shock absorbing material of the Al—Mg—Si seriesaluminum alloy according to the third invention has a thicknessreduction rate for the ruptured cross section of 25% or more whenapplying a tensile test for JIS No. 5 tensile test piece. The thicknessreduction rate for the fractured surface (hereinafter referred to asdraft) is represented by (1-a/a₀)×100, assuming the average thickness atthe ruptured cross section of the tensile test piece as a, whileassuming the original thickness of the tensile test piece as a₀.

[0023] The aluminum alloy according to the third invention is anAl—Mg—Si series aluminum alloy containing from 0.2 to 1.1% of Mg andfrom 0.2 to 1.2% of Si.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024]FIG. 1 is a view showing a cross sectional shape of an extrusionmaterial used in examples;

[0025]FIG. 2 is a view for explaining a longitudinal crushing test foreach of the examples (before and after crushing);

[0026]FIG. 3 is a view for explaining a lateral crushing test for eachof the examples (before and after crushing);

[0027]FIG. 4 is a view showing a cross sectional shape of an extrusionmaterial used in Examples 2 and 3; and

PREFERRED EMBODIMENT OF THE INVENTION

[0028] For obtaining high strength and excellent energy absorbingcharacteristic as the shock absorbing material, it is desirable that themicrostructure of the extrusion material is a fibrous structure (fibrousstructure formed by extrusion is left as it is without recrystallizationduring the heat treatment step subsequent to the extrusion step).Generally, the transition elements such as Mn, Cr and Zr are added tothe Al—Mg—Si series aluminum alloy used for the shock absorbing materialbut addition of such transition elements increases the quenchingsensitivity of the alloy. Further, while the strength of the alloy isimproved as the amount of Mg and excessive Si increases, the quenchingsensitivity is also increased.

[0029] In a case of applying water-cooled press quenching orsolution/quenching treatment, quenching is attained with no problem evenwhen the quenching sensitivity is somewhat sharp and high strength canbe obtained by the subsequent aging treatment. However, if the quenchingsensitivity is increased in the air-cooled press quenching, highstrength can no more be obtained even by the subsequent aging treatment.That is, if the alloy element is added with an aim of improving thestrength, this may rather lower the strength.

[0030] In the first invention, the positive function and effect by theaddition of each of the elements is necessary for obtaining highstrength and excellent energy absorbing property by the aging treatmentafter air cooled press quenching, but it is essential as well not toincrease the quenching sensitivity. With the view point described above,an optimal alloy composition is defined for the impact absorbingmaterial, particularly, undergoing air cooled press quenching.

[0031] Now, the composition of the extrusion material constituting theshock absorbing material according to the first invention is to beexplained.

[0032] Mg, Si:

[0033] Mg and Si are bonded to form Mg₂Si and improve the alloystrength. For obtaining strength required as the frame structuralmaterial of automobiles, Mg has to be added by 0.30% or more. However,if it is added in excess of 0.70%, the quenching sensitivity isincreased and quenching is not attained by the air-cooled pressquenching failing to attain necessary strength. Accordingly, the Mgcontent is defined within a range from 0.30 to 0.70%, preferably, from0.40 to 0.60% and, more preferably, from 0.45 to 0.55%.

[0034] On the other hand, if the amount of excess Si (Si in excess ofthe balanced composition for Mg₂Si, which is defined as “excess Siamount (%)=total Si amount−0.578×Mg amount”) is 0.10% or less, requiredstrength can not be obtained. If it exceeds 0.50%, the quenchsensitivity is increased and quenching is not attained by air-cooledpress quenching and no required strength can be obtained. Accordingly,the content of excess Si is defined as from 0.10 to 0.50%. Within therange for the amount of Mg and the excess Si, the total Si amount isparticularly preferably from 0.5 to 0.7% as a range which can providehigh strength and does not increase the quenching sensitivity not somuch. Further, a more preferred range for the excess amount of Si isfrom 0.22 to 0.40%.

[0035] Mn, Cr, Zr:

[0036] Mn, Cr, Zr have a function of forming a fibrous structure in theextrusion material to improve the cracking resistance and one or morethem is added within a range from 0.10 to 0.40% in total. If theaddition amount of the transition elements is less than 0.10%, nofibrous structure is formed or a thick surface recrystallization layeris formed to cause cracks. If the addition amount exceeds 0.40%,quenching is not attained by air-cooled press quenching and requiredstrength as the frame structural materials for automobiles is notdeveloped. A desired range for each of the elements is from 0.001 to0.35% of Mn, from 0.001 to 0.20% of Cr and from 0.001 to 0.20% of Zr.Further, a more preferred range for the addition amount of thetransition elements in total is from 0.20 to 0.30%. The preferred rangefor each of the elements is from 0.05 to 0.25% of Mn, from 0.001 to0.15% of Cr and from 0.05 to 0.18% of Zr. A further preferred range forthe addition amount of the transition elements in total is from 0.22 to0.28%. The desired range for each of the elements is from 0.10 to 0.20%of Mn, from 0.001 to 0.10% of Cr and from 0.07 to 0.14% of Zr.

[0037] In the alloy according to the present invention, the additionamount of the transition elements is defined as the minimum amountcapable of maintaining the fibrous structure in the extrusion materialby air-cooled press quenching. Therefore, when the solution/quenchingtreatment is applied in off-line instead of press quenching,recrystallization proceeds by the heating upon solution treatment. Thatis, press quenching is an essential condition of forming the fibrousstructure for the alloy composition according the present invention.

[0038] Then, the fibrous structure is desirably formed to the entirecross section of the extrusion material. If the surfacerecrystallization layer is formed, it is necessary that the layer isabout at a depth of 500 μm or less (desirably, 300 μm) from the surfaceof the extrusion material and the thickness of the fibrous structure isabout ½ or more for the entire thickness in a case of an extrusionmaterial of 1 to 5 mm thickness as in the frame structural materials forautomobiles. This is because distortion is concentrated at the grainboundaries of the surface crystallized grains tending to cause crackssince the crystal grain size is greater compared with the fibrousstructure, particularly, the cooling rate is lower in a case of thewater cooled press quenching compared with that in water cooling and theamount of precipitates precipitating to the crystal grain boundariesduring cooling step is increased, so that distortion is concentrated tothe grain boundaries of the surface recrystallized grains tending tocause cracks. If the addition amount for the transition elements such asMn is less than the above mentioned range, it is difficult to keep thethickness for the surface recrystallization layer to 500 μm or less inthe air cooled press quenching.

[0039] Cu:

[0040] Cu has a function of improving the strength of the Al—Mg—Siseries aluminum alloy and improving the stress corrosion crackingresistance, and is added optionally. However, if it is less than 0.10%,the effect is insufficient. On the other hand, the extrudability and thegeneral corrosion resistance are deteriorated if it exceeds 0.40%.Accordingly, the content is preferably from 0.10 to 0.40%. A morepreferred range is from 0.15 to 0.35% and a further preferred range isfrom 0.18 to 0.30%.

[0041] Ti:

[0042] Ti has a function of refining an ingot structure and is addedproperly. However, the refining effect is insufficient if it is lessthan 0.005%, whereas it is saturated to form macro compounds if theaddition amount is more than 0.2%. Accordingly, the Ti content is from0.005 to 0.2%. A more preferred range is from 0.01 to 0.10% and afurther preferred range is from 0.015 to 0.050%.

[0043] Inevitable Impurity:

[0044] Among inevitable impurities, Fe is an impurity contained by thegreatest amount in the raw aluminum metal and, if present in excess of0.35% in the alloy, it would crystallize huge intermetallic compoundsduring casting deteriorates mechanical properties of the alloy.Accordingly, the Fe content is restricted to 0.35% or less. It ispreferably 0.30% or less and, further preferably, 0.25% or less.Further, upon casting the aluminum alloy, impurities are intruded invarious ways such as from raw metals and intermediate alloys of theadditive elements. There are various intruding elements but impuritiesother than Fe scarcely give effects on the properties of alloy ifcontained by 0.05% or less as individual content and 0.15% or less bythe total amount. Accordingly, the content of the impurities is definedas 0.05% or less as the simple element and 0.15% or less as the totalamount. Among the impurities, B is intruded accompanying addition of Tiby about ⅕ of the Ti content in the alloy and a more preferred range is0.02% or less and a further preferred range is 0.01% or less.

[0045] In the first invention, the aging treatment is applied after theair-cooled press quenching for the extrusion material having thecomposition described above. The strength (yield stress) after the agingtreatment is defined as 210 MPa or more as the strength required for theframe structural materials of automobiles. This strength can be obtainedby applying the aging treatment after the air-cooled press quenching forthe extrusion material of the composition described above. However, outof the range of the composition, no desired strength can be developed(energy absorption is lowered as well) or no fibrous structure is formedto deteriorate the energy absorption characteristics. A preferred rangefor the yield stress is 220 MPa or more.

[0046] Desired production conditions in the first invention are to bedescribed. For obtaining products containing the fibrous structure in agreat amount, a homogenization treatment is applied at 450° C. to 550°C. for 2 to 8 hours and the extrusion temperature is defined as 450 to520° C. The aging treatment is conducted appropriately at 160 to 230° C.for 1 to 8 hours. An appropriate cooling rate is from 100 to 300°C./min.

[0047] Then, the composition of the extrusion material according to thesecond invention is to be explained.

[0048] Mg, Si:

[0049] Mg and Si are bonded to form Mg₂Si and improve the alloystrength. For obtaining strength required as the frame structuralmaterial of automobiles, Mg has to be added by 0.20% or more. However,if it is added in excess of 1.1%, the quenching sensitivity is increasedand quenching is not attained by press quenching at low cooling rate. Asthe result, the necessary strength can not be attained. Accordingly, theMg content is defined within a range from 0.20 to 1.1%, preferably, from0.40 to 0.80% and, more preferably, from 0.40 to 0.60%.

[0050] On the other hand, if the amount of Si is 0.20% or less, requiredstrength can not be obtained. If it exceeds 1.2%, the quench sensitivityis increased and quenching is not attained by press quenching at lowcooling rate and no required strength can be obtained. Accordingly, thecontent of excess Si is defined as from 0.20 to 1.2%, preferably, from0.50 to 1.0% and, more preferably, from 0.50 to 0.70%.

[0051] Mn, Cr, Zr

[0052] The reason for defining the numerical values for the range of thecontent is identical with that in the first invention. However,preferably, one or more of them may be added within a range from 0.05 to0.40% for Mn, from 0.05 to 0.20% for Cr and from 0.05 to 0.20% for Zr.More preferably, Mn is from 0.05 to 0.25%, Cr is from 0.05 to 0.15% andZr is from 0.05 to 0.15%. Further, the total amount of addition for thetransition elements is from 0.05 to 0.60%, preferably, from 0.10 to0.40% and, more preferably from 0.2 to 0.3%.

[0053] Cr:

[0054] The reason for defining the numerical values for the range of thecontent is identical with that in the first invention. However, apreferred range is from 0.05 to 0.7% and, more preferred range from 0.10to 0.35%.

[0055] Ti:

[0056] The reason for defining numerical values for the range of thecontent, and the content are identical with those in the firstinvention.

[0057] Surface Recrystallization Layer &P In the Al—Mg—Si seriesaluminum alloy extrusion material described above, it is preferred thatthe fibrous structure of the extrusion material is formed substantiallyfor the entire cross section of the extrusion material. In a case of theextrusion material of 1 to 5 mm thickness as in the frame structuralmaterials for automobiles, it is necessary that the surfacerecrystallization layer is 50% or less for the entire thickness.Preferably it is 30% or less. This is because the recrystallized grainshave a larger crystal grain size compared with the fibrous structure,the amount of precipitates precipitating to the crystal grain boundariesis increased in the course of cooling if the cooling rate is low anddistortion is concentrated to the grain boundaries of the surfacerecrystallization grains tending to cause crackings.

[0058] On the other hand, the Al—Mg—Si series aluminum alloy extrusionmaterial having the constitution and the thickness described above canprovide the fibrous structure over substantially entire cross section ofthe extrusion material (the thickness of the surface recrystallizationlayer is less than 1% of the entire thickness) at the extrusion rate of5 m/min or lower. However, since it takes much time before entering thequenching zone in the press quenching (on-line) at such a low extrusionrate, quenching is difficult making it difficult to obtain a requiredstrength, energy absorption amount and maximum load. Accordingly, it isnecessary that the thickness of the surface recrystallization layer is1% or more for the thickness. It is preferably 5% or more.

[0059] Further, it is preferred that the crystal grain size of thesurface recrystallized grains is 200 μm or less and, further preferably,100 μm or less. This is because distortion is concentrated more tendingto cause cracks as the grain size of the surface recrystallized grainsis larger.

[0060] Then, the composition for the extrusion material according to thethird invention is to be explained.

[0061] Mg, Si

[0062] The reason for defining numerical values for the range of thecontents is identical with that in the second invention. However, apreferred range is from 0.20 to 1.1%, more preferably from 0.40 to 0.8%of Mg and from 0.20 to 1.2%, more preferably from 0.50 to 1.1% of Si.

[0063] Mn, Cr, Zr

[0064] The reason for defining the numerical values for the range of thecontent and the range is identical with that in the second invention.

[0065] Cu:

[0066] The reason for defining the numerical values for the range of thecontent and the range is identical with that in the second invention.

[0067] Ti:

[0068] The reason for defining the numerical values for the range of thecontent and the range is identical with that in the first invention.

EXAMPLE

[0069] Examples of the present invention are to be explained below.

Example 1

[0070] Al—Mg—Si series aluminum alloy billets of the chemicalcompositions shown in FIG. 1-1 were manufactured by melting by DCcasting, and applied with a soaking treatment at 550° C.×4 hr.Successively, by conducting extrusion under the conditions of anextrusion temperature at 500° C. and an extrusion rate of 5 m/min, andpress quenching (blower air cooling (cooling rate: about 190° C./min))at the position just after the extrusion to obtain an extrusion materialof a hollow cross section as shown in FIG. 1 (70 mm for longer side, 50mm for shorter side and 2 mm for thickness). Then, the extrusionmaterial was applied with an aging treatment at 190° C.×3 hr to preparetest specimens. The thickness of the recrystallization layer from theouter surface and the inner surface at the central portion of the longerside and the shorter side for the cross section of the test specimen wasmeasured to determine the thickness as an average value for 8 positions.The result is shown in Table 1. TABLE 1-1 Chemical Composition ofSurface Layer (wt %) Thickness of surface No. Si Mg Fe Cu Mn Cr Zr Zn TiMn + Cr + Zr Excess Si recrystallization layer (one side) 1 0.58 0.540.20 0.20 0.15 tr. 0.09 tr. 0.02 0.24 0.27 200 μm 2 0.66 0.51 0.20 tr.0.15 tr. 0.13 tr. 0.02 0.28 0.37 100 3 0.61 0.51 0.20 0.20 0.25 tr. 0.13tr. 0.02 0.37 0.32 50 4 0.60 0.50 0.20 0.35 0.08 0.08 0.10 tr. 0.02 0.260.31 50 5 0.30 0.52 0.22 0.19 0.15 tr. 0.10 tr. 0.02 0.25 0.00* 200 60.85 0.50 0.23 0.21 0.14 tr. 0.09 tr. 0.02 0.23 0.56* 200 7 0.55 0.25*0.21 0.20 0.16 tr. 0.09 tr. 0.02 0.25 0.41 200 8 0.56 0.75* 0.18 0.200.15 tr. 0.09 tr. 0.02 0.24 0.13 200 9 0.569 0.53 0.19 0.20 tr. tr. 0.09tr. 0.02 0.09* 0.28 600 10 0.57 0.51 0.21 0.21 0.35 tr. 0.09 tr. 0.020.44* 0.28 50

[0071] JIS No. 5 test specimens were sampled from the test materials anda tensile test was conducted according to JIS Z 2241. The result isshown in Table 1-2.

[0072] Further, a static axial crushing test was conducted for the testmaterials. The test material of 200 mm length was axially applied with astatic compressive load by an Amsler tester as shown in FIG. 2, and thetest material was compressed by 100 mm, to obtain a load-displacementcurve, and the maximum load and the absorption energy up to 100 mm weredetermined. The test result is also shown together in Table 1-2. Thecracking resistance was evaluated visually in which those free fromoccurrence of open cracks was evaluated as “◯” and those with occurrenceof open cracks were evaluated as “X”. As the overall evaluation, thosehaving an yield stress (σ_(0.2)) of 210 MPa or more and also excellentin cracking resistance were evaluated as “◯” and those poor in one ofthem were evaluated as “X” TABLE 1-2 Result of Tensile Strength andCompression Test Compression test Tensile test Maximum load AbsorptionCracking Overall No. σ0.2 (Mpa) σB (Mpa) Elongation(%) (kN) energy (J)resistance evaluation 1 228  270 14.1 99.4 3390 ◯ ◯ 2 232  276 14.0 96.23070 ◯ ◯ 3 219  261 14.2 89.2 2980 ◯ ◯ 4 217  252 15.9 88.5 2990 ◯ ◯ 5172* 220 17.8 80.3 2610 ◯ X 6 116* 187 20.2 67.7 2410 ◯ X 7 106* 16714.5 70.5 2500 ◯ X 8 110* 181 19.2 68.1 2530 ◯ X 9 240  277 11.6 1022820 X X 10 136* 197 11.8 72.7 2510 ◯ X

[0073] As shown in Table 1-2, alloys (Nos. 1-4) within the range of thecomposition defined in the present invention show high yield stress andexcellent energy absorbing characteristics (with no cracks, high energyabsorption) through they were obtained by press quenching under aircooling.

[0074] On the other hand, alloys (Nos. 5-10) not satisfying thedefinition for the composition according to the present invention cannot reach the yield stress of 210 MPa which is a required strength asthe flame structural materials of automobiles, or are inferior in thecracking resistance and lowered for the energy absorption although theyield stress is high(No. 9).

Example 2

[0075] Al—Mg—Si series aluminum alloy billets of the chemicalcompositions shown in Table 2-1 were manufactured by melting by DCcasting and applied with a soaking treatment at 550° C.×4 hr.Successively, extrusion was conducted at an extruding temperature of500° C. and at a extruding rate shown in Table 1, and air cooled orwater cooled press quenching in on-line was applied just after extrusionto obtain extrusion materials each of a hollow rectangular cross sectionas shown in FIG. 4 (having 70-80 mm long or side, 54-60 mm shorter sideand 2-5 mm thickness). Air cooling was applied by a blower air coolingat a cooling rate of about 190° C./min, whereas water cooling wasapplied at a cooling rate of about 10000° C./min. Then, an agingtreatment at 190° C. for 3 hours was applied to the extrusion materialsto obtain test materials.

[0076] For the test materials, thickness of the surfacerecrystallization layer (GG layer), crystal grain size in the surfacerecrystallization layer, mechanical properties and cracking resistancein the longitudinal and lateral directions were examined by thefollowing procedures. The results are shown in Table 2-1 and Table 2-2.

[0077] Thickness of Surface Recrystallization Layer:

[0078] Along the cross section in parallel with the extruding direction,4 points on the surface and 4 points on the rear face were selected, thethickness of the recrystallization layer was measured at each of thepoints, the measured values were averaged for those on the surface andthe rearface respectively, and the thickness of the GG layer was definedas the sum of the mean values on the surface and the mean values on therearface.

[0079] Grain Size of the Surface Recrystallization Layer:

[0080] Along the cross section parallel with the extruding direction,four points on the surface and four points on the rearface were measuredby a cutting method from the surface toward the center of the thicknessand the measured values were averaged as the grain size of the GG layer.The grain size was measured from the surface toward the center of thethickness in this test, because the grain size measured along thisdirection has a particular concern with the occurrence of cracks (in thepresent invention, when the thickness of the surface recrystallizationlayer is not sufficiently larger than the grain size of the surfacerecrystallization layer and the measurement for the grain size isdifficult, it is defined as: grain size for the surfacerecrystallization layer=thickness for the surface recrystallizationlayer).

[0081] Mechanical Property;

[0082] JIS No. 5 Test specimens were sampled from the test material anda tensile test was conducted according to JIS Z 2241.

[0083] Longitudinal Compression Property:

[0084] Using a test material of 200 mm length, a static-compression loadwas applied axially as shown in FIG. 2 by an Amsler Tester, which wascompressed to 100 mm thereby obtaining a load/displacement cure, and themaximum load and the absorption energy act 100 mm were determined. Thecracking resistance property was evaluated visually and those free fromthe occurrence of cracks were evaluated as “{circle over (o)}”, thosewith occurrence of fine cracks were evaluated as “◯” and those withoccurrence of open cracks were evaluated as “X”.

[0085] Lateral Compression Characteristic:

[0086] Using a test material of 200 mm length, a static compressive loadwas applied by placing the material horizontally such that the uppersides were situated on upper and lower positions as shown in FIG. 3,which was compressed to 20 mm and the cracking resistance was evaluatedvisually. Those free from the occurrence of cracks were evaluated as“{circle over (o)}”, those with occurrence of fine cracks were evaluatedas “◯” and those with occurrence of open cracks was evaluated as “X”.TABLE 2-1 Extrusion GG layer GG layer Chemical composition (wt %) rateQuenching Thickness thickness GG layer grain size No. Si Fe Cu Mn Mg CrTi Zr (m/min) method (mm) (μm) ratio (%) (μm) example 1 0.61 0.22 0.200.25 0.51 tr. 0.02 0.13 15 Air 2  200  10  50 cooling 2 0.62 0.23 0.200.19 0.51 tr. 0.02 0.14 Air 2  300  30  60 cooling 3 0.64 0.20 0.20 0.150.51 tr. 0.02 0.10 Air 2  500  50  12 cooling 4 0.58 0.20 0.20 0.25 0.50tr. 0.02 0.13 Air 3  300  10  40 cooling 5 0.90 0.20 0.51 0.20 0.60 tr.0.03 0.10 10 water 2  200  10  80 cooling 6 0.91 0.23 0.50 0.36 0.65 tr.0.02 0.12 7 water 5  250  5  30 cooling 7 0.92 0.25 0.49 0.19 0.60 tr.0.02 0.14 water 5 1500  30  90 cooling 8 0.92 0.25 0.49 0.35 0.62 tr.0.02 0.13 6 water 2  60  3  30 cooling comparative 9 0.86 0.27 0.47 0.320.62 0.03 0.02 0.12 3 water 2   0*   0* — example cooling 10 0.60 0.200.07 0.50 tr. 0.02 0.09 15 Air 2  600  60* 130 cooling 11 0.57 0.17 0.20tr. 0.49 tr. 0.03 0.09 Air 2  750  75*  80 cooling 12 0.55 0.20 0.10 tr.0.70 tr. 0.03 tr. Air 2 1000  100* 100 cooling 13 0.55 0.20 0.10 0.090.69 0.04 0.02 tr. Air 3 2700  90* 100 cooling 14 0.91 0.23 0.50 0.090.61 0.04 0.03 tr. 10 water 2 1800  90* 110 cooling 15 0.89 0.27 0.53tr. 0.63 tr. 0.02 tr. water 5 5000  100* 110 cooling 16 0.86 0.27 0.470.32 0.62 0.03 0.02 0.12 7 Air 2  100  5  20 cooling 17 0.60 0.23 0.200.15 0.53 tr. 0.02 0.10 15 Air 2 1000  50  300* cooling

[0087] TABLE 2-2 Result of longitudinal compression test Lateral Maximumcompression Result of tensile test Absorption load Cracking Cracking No.σ0.2 (Mpa) σB (Mpa) Elongation(%) energy (J) (kN) resistance resistanceexample 1 256 207 11.0 2920 75.3 ⊚ ⊚ 2 256 210 14.1 3200 93.1 ⊚ ⊚ 3 270228 12.1 3450 99.4 ◯ ◯ 4 262 210 12.3 10050 181 ⊚ ⊚ 5 305 289 13.6 3210115 ⊚ ⊚ 6 321 290 13.2 21200 661 ⊚ ⊚ 7 315 282 14.1 20100 652 ⊚ ⊚ 8 230201 14.2 2630 68.9 ⊚ ⊚ comp. 9 213 155* 16.3 1260 52.6 ⊚ ⊚ example 10277 225 12.5 3310 92.2 X X 11 280 228 14.0 3000 102 X X 12 262 230 14.02710 99.0 X X 13 270 228 12.5 8250 178 X X 14 295 252 12.7 3010 108 X X15 341 295 12.8 15200 665 X X 16 202 142* 15.6 1150 48.2 ⊚ ⊚ 17 273 23113.1 2760 97.2 X X

[0088] As shown in Table 2-2, those having the GG layer thickness withinthe range defined by the present invention (Nos. 1- 8) cause no cracksand show high yield stress and excellent energy absorbing property (nocracks both in longitudinal and lateral directions, high absorptionenergy and maximum load)

[0089] On the other hand, No. 9 not formed with the surface GG layer hasan yield stress of less than 200 MPa, has low absorption energy andmaximum load, those having the GG layer thickness in excess of the rangedefined by the present invention (Nos. 10-15) are poor in the crackingresistance, No. 16 having an yield stress not reaching 200 MPa althoughhaving the GG layer thickness within the range defined according to thepresent invention has low absorption energy and maximum load, and No. 17with low absorption energy and having the GG layer crystal grain size inexcess of 200 μm is poor in the cracking resistance.

Example 3

[0090] The example of the present invention is to be explained incomparison with a comparative example out of the definition for thescope of the claims of the present invention.

[0091] At first, Al—Mg—Si series aluminum alloy billets (diameter: 155mm) of the chemical composition shown in the following Table 3-1 weremanufactured by melting by the usual method and a homogenizing treatmentwas applied under the condition of about 540° C.×4 hours.

[0092] Then, each of the billets was subjected to extrusion under theconditions at an extrusion temperature of 500° C. and an extrusion rateof 5 m/min, air cooled or water cooled press quenching was conductedon-line just after extrusion to manufacture extrusion materials each ofhollow square cross section as shown in FIG. 4-(1) (a square pipe havingan outer shape of 70×54 mm and a thickness of 2 mm). An artificial agingtreatment was applied under the conditions shown in Table 3-1 to preparetest materials. TABLE 3-1 chemical composition (wt %) Quenching AgingNo. Si Fe Cu Mn Mg Cr Ti Zr method condition 1 0.58 0.18 0.21 0.15 0.54tr. 0.02 0.09 air cooling 190° C. × 3 hr 2 0.61 0.22 0.20 0.25 0.51 tr.0.03 0.13 air cooling 190° C. × 3 hr 3 0.64 0.24 0.20 0.15 0.51 tr. 0.030.10 air cooling 190° C. × 3 hr 4 0.66 0.24 0.20 0.15 0.51 tr. 0.03 0.13air cooling 190° C. × 3 hr 5 0.93 0.25 0.51 0.36 0.67 tr. 0.02 0.13water cooling 210° C. × 3 hr 6 0.93 0.25 0.51 0.36 0.67 tr. 0.02 0.13water cooling 190° C. × 3 hr 7 0.93 0.25 0.51 0.36 0.67 tr. 0.02 0.13water cooling 190° C. × 6 hr 8 0.54 0.20 tr. tr. 0.65 tr. 0.02 tr. aircooling 190° C. × 3 hr 9 0.93 0.25 0.51 0.36 0.67 tr. 0.02 0.13 watercooling 160° C. × 6 hr

[0093] TABLE 3-2 Result of longitudinal compression test Lateralthickness Maximum compression reduction ratio Result of tensile testAbsorption load Cracking Cracking No. (%) σ0.2 (Mpa) σB (Mpa)Elongation(%) energy (J) (kN) resistance resistance 1 40.0 270 228 14.13390 99.4 ◯ ◯ 2 38.2 256 218 14.2 2800 85.8 ◯ ◯ 3 35.1 266 236 12.1 312098.6 ◯ ◯ 4 28.3 259 222 12.8 2930 93.6 ◯ ◯ 5 40.8 314 271 13.2 3530 118◯ ◯ 6 27.6 371 325 13.8 4370 129 ◯ ◯ 7 35.3 358 310 12.9 4010 125 ◯ ◯ 8 24.3* 259 230 15.6 2660 99.3 X X 9  23.1* 362 279 20.3 3230 121 X X

[0094] JIS No. 5 test specimens were sampled from each of the testmaterials, and tensile strength σB, yield stress σ_(0.2), elongation atbreak (elongation strain) ( were measured in accordance with the tensiletest method for the metal material as defined in JIS Z 2241. The averagethickness on the ruptured surface was measured and the thicknessreduction ratio value defined previously was determined. The result isshown in Table 3-2.

[0095] Further a longitudinal and lateral crushing test was conductedfor each of the test materials. In the longitudinal crushing test, astatic compression load was applied in the axial direction by an AmslerTester as shown in FIG. 2, which was compressed to 100 mm to obtain aload-displacement curve and a maximum load and an absorption energy upto 100 mm were determined. The cracking resistance was evaluatedvisually and those free from occurrence of cracks (cracks penetratingthe thickness) was evaluated as “◯” and those with occurrence of opencracks were evaluated as “X”. In the lateral cracking test, the testmaterial was placed horizontally with the longer sides being situated atupper and lower positions and static compressive load was applied tocompress the material down to 20 mm and the cracking resistance wasevaluated visually. Evaluation for the cracking resistance is identicalwith that of the longitudinal crushing test. The result was also showntogether in Table 3-2.

[0096] As can be seen from Table 3-2, any of Nos. 1 to 7 with thethickness reduction ratio of 25% or more is excellent both in thelongitudinal and lateral compression cracking resistance. Nos. 8 and 9with the thickness reduction ratio of less than 25% are poor in thecracking resistance. In addition, while Nos. 1, 3 have the yield stresssubstantially equal with that of No. 8 they show absorption energyhigher by about 20% compared with No. 8. Even in Nos. 2 and 4 having theyield stress lower than that of No. 8, the absorption energy is higherthan that in No. 8 although the maximum load is somewhat lower. No. 5has the yield stress substantially equal with No. 9 but the absorptionenergy is higher compared with No. 9. Nos. 6, 7, which are high in yieldstress, show absorption energy much higher than that of No.9.

[0097] According to the first invention, a shock absorbing materialhaving high strength (yield stress) and showing excellent energyabsorption property can be obtained by conducting press quenching underair cooling followed by the by aging treatment for the Al—Mg—Si seriesaluminum alloy extrusion material. Further, according to the presentinvention, a shock absorbing material which is advantageous in view ofthe dimensional accuracy and the cost compared with that formed by watercooling can be obtained since this is press quenching under air cooling.

[0098] According to the second invention, a shock absorbing materialhaving high strength and showing excellent energy absorption propertycan be obtained by defining the yield stress, the ratio of the surfacerecrystallization layer and the grain size for the Al—Mg—Si seriesaluminum alloy extrusion material.

[0099] According to the third invention, since the thickness reductionratio is defined to 25% or more, a shock absorbing material havingexcellent cracking resistance to the longitudinal compression and thelateral compression and having high absorption energy can be obtained.

What is claimed is:
 1. A shock absorbing material comprising an Al—Mg—Siseries aluminum alloy containing: from 0.30 to 0.70 wt % of Mg; from0.10 to 0.50 wt % of Si in excess of a balanced composition for Mg₂Si;and from 0.10 to 0.40 wt % of one or more of Mn, Cr and Zr in total,wherein said shock absorbing material is of a hollow cross section,comprises a microstructure having a fibrous and has an yield stress of210 MPa or more.
 2. The shock absorbing material as defined in claim 1,wherein said microstructure further comprises surface recrystallizationlayer with the thickness being 10% or less and the thickness of saidfibrous structure is 50% or more for the entire.
 3. The shock absorbingmaterial as defined in claim 1, wherein the total amount of Si is from0.50 to 0.70 wt %.
 4. The shock absorbing material as defined in claim1, further containing from 0.10 to 0.40% of Cu.
 5. The shock absorbingmaterial as defined in claim 1, further containing from 0.005 to 0.2% ofTi.
 6. The shock absorbing material as defined in claim 1, beingobtainable from an aluminum alloy extrusion material and by conductingpress quenching under air cooling just after extrusion and thenconducting aging.
 7. The shock absorbing material as defined in claim 1,being obtainable from an aluminum alloy extrusion material and byconducting press quenching at a cooling rate of 100 to 300° C./min justafter extrusion and then applying aging.
 8. A shock absorbing materialcomprising an Al—Mg—Si series aluminum alloy containing: from 0.20 to1.1 wt % of Mg; and from 0.20 to 1.2 wt % of Si, wherein said shockabsorbing material is of a hollow cross section and comprises amicrostructure having the thickness of a surface recrystallization layerof 1 to 50% for the entire thickness with the balance of a fibrousstructure, having a grain size in the direction of the thickness of thesurface recrystallization layer of 200 μm or less, and having an yieldstress of 200 MPa or more.
 9. The shock absorbing material as defined inclaim 8, containing from 0.40 to 0.80 wt % of Mg and from 0.50 to 1.0 wt% of Si.
 10. The shock absorbing material as defined in claim 8, furthercontaining one or more of Mn, Cr and Zr within a range from 0.05 to0.40% of Mn, from 0.05 to 0.20% of Cr and from 0.05 to 0.20% of Zr. 11.The shock absorbing material as defined in claim 8, further containingfrom 0.05 to 0.7% of Cu.
 12. The shock absorbing material as defined inclaim 8, further containing from 0.005 to 0.2% of Ti.
 13. A shockabsorbing material comprising an Al—Mg—Si series aluminum alloycontaining: from 0.20 to 1.1 wt % of Mg; and from 0.20 to 1.2 wt % ofSi, wherein said absorbing material is of a hollow cross section and hasa thickness reduction ratio of 25% or more at the ruptured cross sectionwhen applying a tensile test.
 14. The shock absorbing material asdefined in claim 13, containing from 0.40 to 0.80 wt % of Mg and from0.50 to 1.1 wt % of Si.
 15. The shock absorbing material as defined inclaim 13, further containing one or more of Mn, Cr and Zr within a rangefrom 0.05 to 0.40% of Mn, from 0.05 to 0.20% of Cr and from 0.05 to0.20% of Zr.
 16. The shock absorbing material as defined in claim 13,further containing from 0.05 to 0.7% of Cu.
 17. The shock absorbingmaterial as defined in claim 13, further containing from 0.005 to 0.2%of Ti.
 18. The shock absorbing material as defined in claim 13, beingobtainable by press quenching just after extrusion and then applyingaging treatment.